Combination thermal and mechanical safety switch for microwave heating oven

ABSTRACT

In a microwave heating oven, a safety switch 301 is disposed in an external position relative to a heating chamber 100. A displacement rod 302, which is made of polyprophylene or the like low dielectric constant plastic substance, is disposed through and across the heating chamber 100. Displacement rod 302 serves as a thermal switch, so that at an abnormally high temperature displacement rod 302 melts down, thereby opening the safety switch 301 and disengaging the microwave generator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to heating ovens; for example,electric or microwave heating ovens. The invention particularly relatesto microwave ovens having mechanical safety switches linked to respondto the open-close action of their front doors, which mechanical switchesalso serve as thermally responsive safety switches. Thus, the switchesboth guard against operation of the microwave when the door is open, andagainst operation when the microwave is overheated.

2. Description of the Related Art

In the conventional microwave heating oven, a box which constitutes aheating chamber, is further enclosed in an external case. Because ofsuch construction, it is difficult to make such a heating oven at a lowcost, or to make such a heating oven easy in manufacture, or to makesuch a heating oven compact. In order to solve such problems, a heatingoven in which the box of the heating chamber serving as the externalcase has been tried. For example, a heating oven wherein the control boxcontaining electric parts is disposed at the side of the heatingchamber, is shown in patent application Sho No. 59-105036, or a heatingoven wherein the control box disposed below the heating chamber, isshown in patent application Sho No. 59-119319.

FIG. 1 shows one of such conventional microwave heating ovens. In FIG.1, the control box 2 containing electric parts is disposed below theheating chamber 1. The front door 3 is disposed at the front opening 1aof such heating chamber 1. The front door 3 has a hook 3a for lockingthe front door 3. And the safety switch 7 is disposed at the positioninvolving such hook 3a on the side wall of the heating chamber.

FIG. 2 shows a control circuit of such a conventional microwave heatingoven. In FIG. 2, the control circuit comprises, a magnetron 31, a highvoltage transformer 32, a high voltage capacitor 33, a diode 34, acooling fan motor 4, a timer control motor 5, and so on. In the safetystandard, for example UL standard, at least two safety switches 7 and 8,responsive to the open-close action of the front door 3 must be includedin the design of the microwave. Further, a short switch 10 also must bedisposed on such electric circuit, to melt down the fuse 9 in caseswitches 7 and 8 become stuck closed when front door 3 is open.Furthermore, the conventional microwave heating oven has a thermalswitch 11 which is, for example, a fuse switch or a bimetal switch, soas to break off the electric circuit for making microwave 30 and thecooling fan motor 4 in case of abnormal high temperature or fire troubleof food. In practical design of the microwave heating oven, the positionof the thermal switch is determined after designing of a passage of theexhaust gas from the cooked food by trial and error of the experiment.

As improvement toward simplicity of the configuration of the heatingoven proceeds, however, the safety switch responding to the open-closeaction of the front door must be disposed at the position to be showeredby the microwave, which is shown in FIG. 1. So that, such a heating ovenhas the following problems. First, shower of microwave destroys thesafety switch, second at receiving such microwave shower, wires ofcontrol circuit radiate electric noise which a badly affect thetelevision set or the radio.

Further, in a microwave heating oven such as shown in patent applicationSho No. 59-119319, the walls of the heating chamber serves as theexternal case, and so the heat is conveyed toward external part easily.Therefore, there is a strong demand for the quick responding thermalswitch which can break down the electric circuit in a short time.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a microwave heatingoven having safety swiches, at least one of which serves dually as athermal switch.

A microwave heating oven in accordance with the present inventioncomprises,

a heating chamber for containing and heating a heating object therein,

a microwave generator for generating microwave for heating the heatingobject,

a front door for covering a front opening part of the heating chamber,

a safety switch which is disposed behind a microwave-shield and is forswitching off the operation of the microwave generator, responding toopening action of the front door,

physical displacement means for transmitting the opening action of thefront door to the safety switch, disposed across and through the heatingchamber.

As a result of the above-mentioned configuration, the safety switchesare safely shielded from the microwave and hence are not broken by theradiation of the microwave, and on the other hand the switch is quicklyopened in the event of abnormally high temperatures of fire trouble.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the cross-sectional side view of the conventional microwaveheating oven.

FIG. 2 is the drawing of the electric circuit for making microwave inaccordance with the conventional microwave heating oven.

FIG. 3 is a cross-sectional side view of a microwave heating oven inaccordance with the present invention.

FIG. 4 is a cross-sectional side view of the microwave heating oven inaccordance with FIG. 3, showing a case of a fire trouble and action of asafety switch serving as a thermal switch..

FIG. 5 is a cross-sectional plan view of the microwave heating oven ofFIG. 3.

FIG. 6 is a perspective view of a microwave heating oven in accordancewith the present invention.

FIG. 7 is a circuit diagram of an electric circuit of the microwaveheating oven in accordance with the present invention.

FIG. 8 is a perspective view of a lock mechanism and switches forresponding to a front door in accordance with FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of a microwave heating oven in accordance withthe present invention is shown in FIG. 3, FIG. 4, FIG. 5, FIG. 6, FIG. 7and FIG. 8.

As shown in FIG. 3, a box made of a microwaveshielding substance, suchas metal, and constituting a heating chamber 100 of such embodiment isdesigned to serve also as an external case, so as to make the size ofthe microwave heating oven compact and reduce cost. Panels 101, 102,103, 104, 106, 107 and so on, which are for example stainless steel, aremade in box shape by known bending and calking and the like. A controlbox 200 in which a magnetron 201, a high voltage transformer 202, a highvoltage capacitor 203, a cooling fan motor 204, and so on are contained,is disposed below the heating chamber 100. At a front opening window ofthe heating chamber 100, a front door 110 is connected rotatably to thepanel 107 by a known mechanism (which is not shown in the drawings). Anda front part of the control box 200 is covered by a control panel 120. Atimer 207 for controlling the cooking time, (namely radiation time ofmicrowave,) and a knob of the timer 208 are disposed at the front faceof the control panel 120.

As shown in FIG. 6 which is a perspective view of the heating oven inaccordance with FIG. 3, the front door 110 has a projection part 110band a hook part 110c on its inner face. The projection part 110b turnson a safety switch 206 disposed on the control box 200 when the frontdoor 110 is closed, and turns off the safety switch 206 when the frontdoor 110 is opened. And the hook part 110c is caught to be locked andreleased to be unlocked by a known mechanism in the control box 200,responding to the close action and open action of the door knob 110d ofthe front door 110 (the lock mechanism is not shown in the drawings),respectively.

The control panel 120 has openings 120a and 120b, respectively forreceiving the projection part 110b and the hook part 110c of the frontdoor 110. And the lock mechanism serves as another safety switch. Thesemechanism and the safety switches are shown in detail in FIG. 8 andexplained later.

A rear cover 105 is fixed on the rear part of the heating chamber 100,and the space 300, which is enclosed by the panel 102 of the heatingchamber 100 and the rear cover 105, constitutes the exhaust passage ofthe heating oven to exhaust the vapor or the like from the food 130 ofheating object by the absorption of the cooling fan 204a. On the innerface of the rear cover 105, a safety switch 301 which turns off or onresponding to the open-close action of the front door 110, is disposed.A displacement rod 302, which transmits the open-close action of thefront door 110 to the safety switch 301, is disposed through the upperpart of the heating chamber 100. And also, it is supported at both endparts in a direction shown by arrow marks A and B in FIG. 3 by bushings304 and at inbetween parts by guide holders 305. The guide holders 305are held by the panels 102 and 103 of the heating chamber 100. Thedisplacement rod 302 is always charged by a spring of safety switch 301(which is not shown in figures) so as to be pushed out of the panel 107.When the front door 110 is opened, the displacement rod 302 slides inthe direction arrowed A by the force of the spring of the safety switch301 and is pushed out of the panel 107, so that the right end 302b ofthe displacement rod 302 is separated from the safety switch 301, andthe safety switch 301 is opened. And when the front door 110 is closed,the inner face 110a of the front door 110 contacts with the left end302a of the displacement rod 302, and so, the displacement rod 302 iswithdrawed in the direction arrowed B, and the safety switch 301 isclosed. Accordingly, the safety switch 301 is responding to theopen-close action of the front door 110.

On the bottom of the heating chamber 100, a magnetron 201 whichgenerates the microwave, and a stirrer blade 209 which radiates themicrowave equally on the food 130, are disposed. The food 130 ispartitioned from the magnetron 201 and the stirrer blade 209 by apartition 140 made of ceramics or the like insulating substance.

The safety swich 301 which is mentioned above, also serves as a thermalswitch. The thermal switch is for sensing the abnormal high temperatureor heat in the heating chamber 100 caused by the fire trouble of thefood or the like, and for cutting off the electric circuit of microwavegenerating circuit 250 and the cooling fan motor 204, so that theopening of the thermal switch 301 prevents propagation of heat to theoutside of the heating oven in an early stage.

The constitution and operation of the thermal switch is as follows.

The displacement rod 302 is made of low dielectric loss materials suchas polypropylene, so that it does not ignite at normal heatingtemperatures of the microwave, but it easily changes its' shape atabnormally high temperature caused by the fire trouble of the food orthe like. Accordingly, in case of the fire trouble of the food which isshown in FIG. 4, the center part of the displacement rod 302 changes itsshape, and does not withstand the force of the spring of the safetyswitch 301 (which is not shown in figures). So, the spring returns tonormalcy and the right end 302b is pushed in the direction designated byarrow mark A. Then, the safety switch 301 is turned off and cuts off theelectric circuit of making microwave 250 and the cooling fan motor 204.

FIG. 5 is a cross-sectional plan view of the heating oven in accordancewith FIG. 3.

On the center of the heating chamber 100, the food 130 of heating objectis disposed. The displacement rod 302, the guide holder 305 and thesafety switch 301 are disposed on the right end part of the heatingchamber 100. And also these switching mechanisms is disposed on theupper part of the heating chamber 100, so the heating chamber 100 ishardly restricted by these switching mechanisms. When the microwaveradiated the food 130 in cooking, the vapor or the like are produced.Such vapor is drafted through the punched holes 102a of the panel 102 inFIG. 5 and exhausted out of the heating oven by the cooling fan 204a inFIG. 3. The exhaust passage is shown by the arrow marks.

FIG. 7 shows an example of the electric circuit of this embodiment ofthe microwave heating oven. In the electric circuit the above-mentionedat least two safety switches 206 and 301 to comply with the safetystandard are provided. The safety switches 206 and 301 are responsive tothe open-close action of the front door 110. Further, a short-circuitingswitch 280 is also provided in such electric circuit, to turn on in caseof failure of the safety switches 206 and 301, so as to melt down a fuse260. Incidentally, 270 designates a main switch. As shown in FIG. 8, theshort-circuiting switch 280 is provided to connect two power lines atthe part beyond other safety switch 206, 301 and a fuse 260 in paralelwith loads in this apparatus, and serves as lock mechanism of the frontdoor 110.

In such electric circuit, a timer motor 207a of a timer 207 whichcontrols the heating time, the cooling fan motor 204 and an electriccircuit for making microwave are connected in paralel. Further, themicrowave generating circuit 250 comprises the magnetron 201, the highvoltage transformer 202, the high voltage capacitor 203, the diode 210and so on.

As mentioned above, the present invention uses a thermal switch 301 asone of safety switches, and the safety switches are disposed at aposition thermally isolated from the heating chamber, so that the safetyswitches of the present invention have high reliability. The electriccircuit of the present invention has simplicity, and the microwaveheating oven can be made without losing the compactness and the lowcost, in comparison with the conventional compact type microwave heatingoven.

What is claimed is:
 1. A microwave heating oven comprising:(a) heatingchamber means with aperture therein, adapted for containing and heatingan object placed therein; (b) heating chamber door means capable ofbeing connected to said heating chamber means so as to cover saidaperture; (c) microwave generator means connected to said heatingchamber means so as to irradiate and thereby heat said object; (d)safety switch means connected to said microwave generator means so thatopening said switch means de-energizes said microwave generator means;(e) physical displacement means connected between said door means andsaid safety switch means;(1) said displacement means being connected tothat closing said door means causes said displacement means to closesaid switch means and that opening said door means causes saiddisplacement means to open said switch means, (2) said displacementmeans being disposed thermally proximate to said chamber means so thatoverheating of said chamber means causes said displacement means tobecome overheated, (3) said displacement means being made of a thermallyresponsive material so as to be rigid at normal operating temperaturesof said heating chamber means and yet flexible at overheatedtemperatures of said heating chamber means, (4) said displacement meansbeing connected between said door means and said switch means so thatwhen said displacement means becomes flexible, said switch means iscaused to open.
 2. An oven as in claim 1 wherein said switch means isshielded from said heating chamber means by microwave shield means, sothat said switch means is thermally insulated from said chamber means.3. An oven as in claim 2 wherein said chamber means is made frommicrowave shielding material.
 4. An oven as in claim 2 wherein saidswitch means is disposed near one side of said chamber means opposite toa side thereof adjacent said door means, so that said displacement meanspasses tangential to one or more other sides of said chamber means as itpasses between said door means and said switch means.
 5. An oven as inclaim 4 wherein said displacement means is made of a low dielectric losssubstance.
 6. An oven as in claim 4 wherein said displacement means ismade of polyprophylene.
 7. An oven as in claim 4 wherein said switchmeans comprises spring loaded electrical switch means, and saiddisplacement means comprises a bar of temperature responsive, lowdielectric loss material; so that closing of said door means pushes saiddisplacement means so as to close said spring-loaded switch means, andopening of said door means allows said spring-loaded switch means topush said displacement means back to its original position and therebyopen said switch means, and that upon said displacement means becomingflexible, as when said chamber means overheats, said spring-loadedswitch means opens, thereby de-energizing said microwave generatormeans.
 8. An oven as in claim 4 wherein said displacement means islocated tangential to a side of said chamber means near to the top ofsaid chamber means.
 9. An oven as in claim 4 wherein said switch meanscomprises a plurality of switches, and said displacement means comprisesa plurality of displacement bars activating said plurality of switches.